In a cassette type gas appliance such as a cassette type cooking gas stove, a cassette type gas stove or the like, the fuel gas can be successively supplied from the fuel gas cassette to the burner without any problem at normal temperatures and the fuel gas in the cassette can be easily exhausted so long as the gas appliance is of a low-calorie type which is lower than 1800 kcal/hr in caloric force.
On the other hand, in the case of a high-calorie gas appliance where the caloric force is not lower than 1800 kcal/hr, the amount of vaporizing liquefied gas in the cassette increases with increase in gas supply to the burner. As the amount of vaporizing liquefied gas in the cassette increases, vaporization latent heat increase and when the vaporization latent heat exceeds the heat capacity of the cassette casing and the liquefied gas therein and the quantity of heat from surroundings, the temperature of the liquefied gas in the cassette lowers, which lowers the equilibrium gas pressure. When the equilibrium gas pressure lowers, a required amount of vaporized gas cannot be supplied to the burner from the cassette, which lowers the caloric force at the burner to make trouble in use of the gas appliance and makes it difficult to exhaust the cassette of the liquefied gas therein.
That is, when the caloric force is weakened in response to reduction in gas supply due to temperature drop of the fuel gas cassette, the user will consider the cassette to be exhausted and attempt to replace the fuel gas cassette. However when the user shakes the removed cassette, he or she will know that there remains some liquefied gas in the cassette. When the temperature of the cassette is elevated to the room temperature, gas supply becomes feasible again but the temperature of the cassette will drop soon to cause a shortage of fuel supply. Thus it is troublesome to exhaust the cassette of liquefied gas. Further the fact that good combustion cannot be obtained though there remains liquefied gas in the cassette causes the gas appliance and/or the fuel gas cassette to seem defective and damages reliability of the products.
Thus it is most preferred that the gas appliance burns at a predetermined high-calorie so long as there remains any amount of liquefied gas in the cassette and is quenched with its caloric force abruptly weakened when the cassette is exhausted.
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 55(1980)-25757, there has been known a structure in which the fuel gas cassette is heated by heat of the burner through a heat transfer plate. That is, in the structure, the heat transfer plate is disposed with its one part positioned near the burner and its another part in contact with a fuel gas cassette set to the gas appliance so that heat of the burner is transferred to the cassette to suppress temperature drop of the liquefied gas in the cassette due to vaporization latent heat, thereby accelerating vaporization of the liquefied gas to ensure sufficient gas supply to the burner and to ensure exhaustion of the cassette.
However this approach is disadvantageous in that it is difficult to design the heat transfer plate from the viewpoint of how much heat should be transferred to the cassette. When the gas appliance is used in an elevated temperature area in summer, heat supply to the cassette from the air increases and at the same time heat dissipation during heat transfer through the heat transfer plate reduces. Accordingly when the heat transfer through the heat transfer plate is large, the cassette can be overheated and the internal pressure of the cassette can become abnormally high. Accordingly the heat transfer plate should be designed so that the cassette cannot be overheated even under such a high temperature condition.
On the other hand, when the gas appliance provided with a heat transfer plate designed to meet the above requirements is used under a low temperature condition in winter, heat supply to the cassette through the heat transfer plate becomes insufficient and gas supply to the burner becomes insufficient due to temperature drop of the cassette caused by the latent heat upon vaporization of the liquefied gas, which results in poor caloric force at the burner. Further when the amount of liquefied gas remaining in the cassette is small, heat capacity of the liquefied gas in the cassette becomes smaller. That is, the smaller the amount of liquefied gas remaining in the cassette is, the larger the temperature drop is.
As can be seen from the description above, the approach where a part of heat of combustion at the burner is transferred to the cassette through a heat transfer plate to suppress temperature drop of the cassette can accomplish the object only under a particular condition (which will be described with reference to FIGS. 14 to 16 later). That is, little heat is supplied to the cassette through the heat transfer plate for a predetermined time after initiation of combustion, and heat supply to the cassette through the heat transfer plate is not stabilized until a predetermined time (e.g., 5 to 7 minutes) elapses. In normal use of a gas appliance, the time for which a high-calorie is required is often shorter than such an initial time and accordingly if the amount of liquefied gas remaining in the cassette is small, an abrupt temperature drop occurs, which results in shortage in caloric force and difficulties in exhausting the cassette of liquefied gas.
Another approach to prevent temperature drop of the liquefied gas in response to gas supply to the burner due to vaporization latent heat involves, as disclosed for instance in Japanese Unexamined Patent Publication No. 54(1979)-123726, use of vaporization accelerating material in the form of latent heat material disposed inside or on the cassette. The latent heat material generates heat of solidification which is supplied to the cassette to suppress temperature drop of the cassette.
This approach gives rise to problem that it is difficult to supply heat from the latent heat material stably for a long time. That is, in the case of a gas appliance where the caloric force is high and gas consumption is large, cooling rate of the liquefied gas due to the vaporization latent heat is large since the amount of vaporization is large. Accordingly even if heat supply from the latent heat material through the cassette wall is initially sufficient, heat inside the latent heat material is not sufficiently transferred outward through the area of contact if heat transfer and convection inside the latent heat material are not sufficient, which can result in shortage of heat transferred to the cassette and temperature drop of the cassette though the heat capacity of the overall latent heat material is sufficient. Thus vaporization accelerating effect cannot be obtained satisfactorily. Especially when the gas appliance is used with a small amount of liquefied gas remaining in the cassette, the temperature drop is sharp and the above phenomenon is remarkable.
Another approach of heating the fuel gas cassette involves use of a heat transfer plate which is in contact with the cassette and supplies heat obtained by heat exchange from the surrounding air to the cassette, thereby suppressing temperature drop of the cassette as disclosed, for instance, in Japanese Unexamined Patent Publication No. 54(1979)-100880.
In this approach, the quantity of heat supplied to the cassette through the heat transfer plate greatly depends upon the environmental temperature and there is a problem in supplying a stable quantity of heat for a long time.
As described above, in the approach where heat of combustion at the burner is supplied to the gas cassette through a heat transfer plate, the quantity of heat to be supplied should be limited not to bring the cassette into an overheated state even under a high temperature condition of use. Accordingly, it takes 6 to 7 minutes for the temperatures of the parts of the heat transfer plate to attain equilibrium after ignition of the burner, and during this period, heat supply to the cassette through the heat transfer plate is insufficient (See FIG. 20). In the approach where the cassette is heated by use of a latent heat material, it has been found that though a sufficient quantity of heat can be initially supplied to the cassette through supply of sensible heat and latent heat of fusion of the latent heat material, heat transfer from the inside of the latent heat material is reduced after long use and the temperature of the cassette tends to drop. (This will be described later with reference to FIGS. 14 to 16) It is considered that the heat transfer plate for heat exchange has the similar tendency.
When a fuel gas cassette is set to a gas appliance and the gas appliance starts burning at a high calorie (e.g., 2500 kcal/hr), the temperature of the cassette drops and the caloric force lowers as time lapses. In order to maintain a desired caloric force, the cassette should be kept at not lower than 6.degree. C. at the lowest, and preferably not lower than 8.degree. C. Though substantially the same cassette temperature is required irrespective of the desired caloric force, a lower caloric force can be maintained even if the cassette temperature is somewhat lower. Thus, in order to keep the current fuel gas of butane burning rate high, the temperature of the cassette must be kept not lower than the above values.
In view of the foregoing observations and description, the primary object of the present invention is to provide a vaporization acceleration device for a high-calorie gas appliance which can supply proper amount of heat to the cassette and suppress temperature drop of the cassette irrespective of temperature of the atmosphere of use and irrespective of whether the fuel gas start burning or has been burning a long time, thereby accelerating vaporization of the liquefied gas so that the caloric force can be maintained high and the cassette can be exhausted of liquefied gas therein. The vaporization acceleration device of the present invention has been made on the basis of heat supply properties of a heat transfer plate which transfers a part of combustion heat to the cassette and a heat accumulator or heat exchanger member which is in contact with the cassette and selectively supplies heat according to the temperature difference.